Add butter and sugar into a bowl, and mix until the mixture becomes white (If it's hard to blend, add a little oil).,Add egg and remaining vegetable oil into the bowl, and mix some more.,Sift the ingredients.,Add milk and dry ingredients alternately in 2 batches to Step 2, and mix well.,Add chocolate chips to the batter.,Save some for scattering on top.,Pour the batter into muffin cups, and sprinkle the remaining chocolate.,Bake in the oven for 25 ~ 30 minutes at 170C (340F).,Done.,______